[gmx-users] eelctric field

[Christian Blau]

Hello Amin,


Though the forces are present during the simulation the additional work on the system that is exerted via the electric 
field is not output as an additional energy term in GROMACS.

I believe the pressure deviations that you observe are due to two different effects:

     - the pressure fluctuates much more than energies, like coulomb interactions, etc., especially in small systems. 
The question here would be if the deviation between the perturbed and unperturbed system that you observe is larger than 
the deviation between two unperturbed systems

     - the electric field indirectly influences the pressure. I would expect this effect to be usually small, but if you 
imagine a system consisting only of the same number of positively and negatively charged ions, applying the electric 
field along z-direction will cause half of them to move upwards and the other half downwards along the z-direction, 
which will in turn affect the pressure.


Best,

Christian

On 11/13/19 3:39 PM, Amin Rouy wrote:
> Dear Christian,
>
> Thank you for your reply. However, still it is not fully clear to me. Lets
> only talk about gmx energy.  How it happens that I get from gmx energy
> similar values of energies (LJ, columbs, total energies) while a different
> pressure, with and without applied filed? The total energy should be
> different as a result of an additional energy due to applied filed on each
> atom.
>
> On Wed, Nov 13, 2019 at 3:10 PM Christian Blau <blau at kth.se> wrote:
>
>> Hello Amin,
>>
>>
>> When you apply an electric field during the simulation by setting the mdp
>> parameters like E-x =... as you describe,
>> nothing more happens than that each atom experiences an additional force
>> that is "atom charge * field strength". The
>> energy terms or virial (that then would relate to pressure coupling) are
>> not influenced by this. If these terms differ
>> between a simulation with electric field and one without, then only
>> because the system as a whole responds to the
>> applied electric field.
>>
>> gmx potential on the other hand is an analysis tool that is per se
>> unrelated on how you chose to apply an electric field
>> during your simulation or not. However, when applying an electric field,
>> you would expect a different result from an
>> analysis with gmx potential than without electric field, because your
>> system will respond to the applied electric field.
>>
>>
>> Best,
>>
>> Christian
>>
>> On 11/13/19 2:44 PM, Amin Rouy wrote:
>>> Hi,
>>>
>>> I am using gromacs 5.0.4. I apply a uniform electric field in x direction
>>> of my box (no walls),
>>> E-x= 1  10  0, and simulation runs without error.
>>> I notice that the values of electrostatic interactions I get from the
>>> simulation is the same as without the an electric field. Which is
>> strange,
>>> but according to previous questions in the gromacs forum this is true:
>>> ''gmx potential only calculates the electric field and potential of the
>>> system. It does not take into account the applied field.''
>>>
>>> Now, my question is are the other quantities obtaining from gmx energy
>> take
>>> into account field effect? e.g. pressure of the system.
>> --
>> Gromacs Users mailing list
>>
>> * Please search the archive at
>> http://www.gromacs.org/Support/Mailing_Lists/GMX-Users_List before
>> posting!
>>
>> * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists
>>
>> * For (un)subscribe requests visit
>> https://maillist.sys.kth.se/mailman/listinfo/gromacs.org_gmx-users or
>> send a mail to gmx-users-request at gromacs.org.
>>